1. Field of the Invention
The present invention relates to a multi-piece solid golf ball including a core and a multi-layer cover, and more particularly to a golf ball enabling a golfer who has a rather low club head speed to drive the ball a long distance.
2. Description of the Related Art
U.S. Pat. No. 5,846,141 discloses a golf ball having an initial velocity in excess of 255 feet/s (77.7 m/s) when tested under the conditions set forth in the Royal and Ancient Golf Club of St. Andrews (RandA) and the United States Golf Association (USGA) and traveling less than 296.8 yards (271.4 m) in carry and run when tested under the conditions set forth in RandA and USGA.
The golf ball described in U.S. Pat. No. 5,846,141 has a core and a cover and an initial velocity in excess of 255 feet/s (77.7 m/s) when tested under the conditions set forth in the applicable rule established by USGA, and travels less than 296.8 yards in carry and run when tested under the conditions set forth in the applicable rule. According to the publication, since the golf ball has an initial velocity in excess of that prescribed by RandA and USGA, the golf ball enables a golfer who has a rather low club head speed to drive the ball a long distance.
However, U.S. Pat. No. 5,846,141 does not specifically disclose how the initial velocity of the golf ball is increased, and those skilled in the art have encountered difficulty in embodying a golf ball having an increased initial velocity from the publication.
In view of the foregoing, it is an object of the present invention to provide a multi-piece solid golf ball having an increased initial velocity implemented through specification of conditions with respect to dimples, core hardness, and cover hardness, among others, to thereby enable a golfer who has a rather low club head speed to drive the ball a long distance.
To achieve the above object, the present invention provides a multi-piece solid golf ball comprising a core and a multi-layer cover, the golf ball satisfying the following conditions (1) to (5):
(1) the number of dimples is 360 to 492;
(2) total volume enclosed by the dimples as expressed as a percentage of the volume of the golf ball, VR, is 0.715% to 0.825%;
(3) total surface area of the dimples as expressed as a percentage of the surface area of the golf ball, SR, is not less than 70%;
(4) value obtained through subtraction of JIS C hardness of a surface portion of the core from average JIS C hardness of the cover is 0 to 20; and
(5) initial velocity is in excess of 77.7 m/s (255 ft/s).
In the present invention, the VR (volume ratio), the SR (surface ratio), the average JIS C hardness of the cover, and the initial velocity have the following meanings.
[VR]
VR (%) is expressed as below.
[Expression 1]      V    R    =                    V        S                              4          3                ⁢        π        ⁢                  xe2x80x83                ⁢                  R          3                      xc3x97    100  
(where VS is total space volume of dimples, each having a dimple space volume VP, and R is the radius of the golf ball (an imaginary sphere). The dimple space volume VP is defined as the volume of a space enclosed by a dimple surface and a plane surrounded by an edge of the dimple.)
VS appearing above in Exp. 1 is expressed below by Exp. 2. VS obtained from Exp. 2 is substituted into Exp. 1 to thereby obtain VR.
[Expression 2]      V    S    =                              N          1                ⁢                  V          P1                    +                        N          2                ⁢                  V          P2                    +      …      +                        N          n                ⁢                  V          Pn                      =                  ∑                  I          =          1                n            ⁢              xe2x80x83            ⁢                        N          i                ⁢                  V          Pi                    
(where VP1, VP2, . . . , VPn each represent the volume of a dimple, the dimples being of different shapes. N1, N2, . . . , Nn each represent the number of dimples having the respective one of volumes VP1, VP2, . . . , VPn. n is an integer equal to or greater than 1.)
A method for obtaining the dimple space volume VP will next be described with respect to a dimple having a circular shape as viewed from above. Referring to FIG. 1, an imaginary spherical surface 5 having the diameter of the ball is formed on a dimple 4, and an imaginary spherical surface 6 having a diameter 0.16 mm smaller than the diameter of the ball is formed as well. A dimple edge 10 is defined as a collection of intersections 9, each of which is an intersection of the imaginary spherical surface 5 and a tangent 8 to the dimple 4 at an intersection 7 of the imaginary spherical surface 6 and the dimple 4. Since an edge portion of the dimple 4 is usually rounded, the dimple edge 10 is thus defined for clarity. Referring to FIGS. 2 and 3, the dimple space volume VP of a dimple space 12 is obtained on the basis of a plane (a circle having a diameter Dm) 11 surrounded by the dimple edge 10 and a distance (a dimple depth Dp) between the plane 11 and the bottom of the dimple space 12. When dimples of a single type are involved, the product of the dimple space volume VP and the number of dimples is obtained. When dimples of two or more types are involved, the product of the dimple space volume VP and the number of dimples is obtained for each type of dimples. The thus-obtained product(s) is used to obtain the total space volume of dimples VS according to Exp. 2. The thus-obtained VS is used to obtain the total volume enclosed by dimples as expressed as a percentage of the volume of a golf ball, VR, according to Exp. 1.
[SR]
SR(%) is expressed as below. A dimple edge is defined as in the case of VR described above.
[Expression 3]      S    R    =            S      S              4      ⁢      π      ⁢              xe2x80x83            ⁢              R        2            
(where SS is total surface area of dimples, and R is the radius of a golf ball (an imaginary sphere). The surface area of a dimple is defined as the area of a dimple surface below the plane surrounded by the dimple edge.)
[Average JIS C hardness of the Cover]
The average JIS C hardness of the cover is a value obtained by dividing the sum of the product, for each layer, of JIS C hardness and thickness, by the sum of the thicknesses of the layers. In an example case in which the cover has two layers, the average JIS C hardness of the cover is calculated by the following formula.
Average JIS C hardness of the cover=(Axc3x97X+Bxc3x97Y)/(X+Y), 
where A is the JIS C hardness of the first cover layer, X is the thickness of the first cover layer, B is the JIS C hardness of the second cover layer, and Y is the thickness of the second cover layer.
In an example case in which the cover has three layers, the average JIS C hardness of the cover is calculated by the following formula.
Average JIS C hardness of the cover=(Axc3x97X+Bxc3x97Y+Cxc3x97Z)/(X+Y+Z) 
where A is the JIS C hardness of the first cover layer, X is the thickness of the first cover layer, B is the JIS C hardness of the second cover layer, Y is the thickness of the second cover layer, C is the JIS C hardness of the third cover layer, and Z is the thickness of the third cover layer.
[Initial Velocity]
Initial velocity is measured on an initial velocity meter similar to a drum-rotation-type initial velocity meter of USGA approved by RandA. A ball is conditioned for a minimum of 3 hours at 23xc2x11xc2x0 C. and is then tested in a room conditioned to 23xc2x12xc2x0 C. The ball is struck by a club head (a striking mass) of approximately 250 lbs (113.4 kg) at a striker velocity of 143.8 ft/s (43.83 m/s). A dozen balls are struck four times each. Time required to pass through a distance of 6.28 ft (1.88 mm) is measured to thereby calculate initial velocity. The cycle is completed in approximately 15 minutes.
Other and further objects, features and advantages of the present invention will be more apparent from the following description.